1. Field of the Invention
The invention relates to a light intensity measurement system and more particularly, to a light intensity measurement system of a projection illumination system for microlithography.
2. Description of Relevant Art
Such systems are in use in wafer steppers or scanners of the ASM-L Company, with optics of the Carl Zeiss Company, Oberkochen, Germany. International patent publication WO 97/31298 is a related publication. Such systems are also known from European Patent 0 500 393 B1 (FIG. 134 ).
The measurement of light intensity--and thereby the energy, after integration over time--serves for the precise exposure of photoresists, and also as an input magnitude for control circuits by means of which the effects of the light flux on the optical elements (lens heating, thermal lenses) are stabilized.
Problems of measurement accuracy arise with increasing requirements for the accuracy of the whole exposure system and process, and with increasing variability of the whole system, particularly at the exit of the glass rod used in the system, with the variable adjustment of a reticle masking diaphragm whose diaphragm lamellae give rise to considerable reflections back into the glass rod.